1. Field of the Invention
The present invention relates to dot matrix impact line printers, and more particularly to print hammer mechanisms for use therein in which flexible hammer springs normally held in a retract position by a magnetic structure are selectively released to effect impact printing via dot printing impact tips mounted on the free ends of the hammer springs.
2. History of the Prior Art
It is known to provide in a dot matrix line printer a reciprocating shuttle containing a hammer bank in which a plurality of elongated, resilient, generally parallel hammer elements having dot impacting tips at the free ends thereof are selectively released from retracted positions so as to impact an ink ribbon against a platen supported print paper as the shuttle reciprocates relative to the print paper. Such an arrangement is shown in U.S. Pat. No. 3,941,051 of Barrus et al, issued March 2, 1976 and commonly assigned with the present application. In the Barrus et al patent, the hammer bank employs a print hammer mechanism which forms a generally C-shaped magnetic circuit between the opposite fixed and free ends of the hammer elements. The magnetic circuits include a common permanent magnet to which the hammer elements are coupled at their fixed ends, a common magnetic return path coupled to the permanent magnet opposite the hammer elements and a plurality of pole pieces, each of which extends outwardly from the magnetic return path so as to terminate in a pole tip facing the free end of the hammer element. Flux from the permanent magnet normally pulls the hammer element out of a neutral position and into a spring-loaded retract position against the pole piece. Each time a coil surrounding the pole piece is momentarily energized, the attracting force of the permanent magnet is overcome long enough to release the hammer element from the retract position and send it flying in the direction of the ink ribbon and print paper. Following impacting of the dot printing tip against the ribbon and paper, the hammer element rebounds back into the spring-loaded retract position in preparation for the next energization of the coil.
The print hammer mechanism shown in the Barrus et al patent utilizes a single pole piece with each hammer spring. It has been found that the performance of such mechanisms can be improved by adding a second pole piece as shown, for example, by U.S. Pat. No. 4,233,894 of Barrus et al which issued Nov. 18, 1980 and which is also commonly assigned with the present application. In the print hammer mechanisms of U.S. Pat. No. 4,233,894, the hammer spring contacts the primary pole piece and at the same time forms an air gap with a secondary pole piece disposed between the primary pole piece and the fixed end of the hammer spring. The air gap formed by the secondary pole piece and the additional flux path provided by the secondary pole piece combine to improve both the release and retract characteristics of the print hammer mechanism. An alternative arrangement utilizing two different pole pieces with each hammer spring and realizing similar advantages is shown in U.S. Pat. No. 4,258,623 of Barrus et al which issued Mar. 31, 1981 and which is also commonly assigned with the present application.
The print hammer mechanisms shown in the various Barrus et al patents have been found to function reliably and effectively for printer applications utilizing speeds of up to 600 lines per minute and greater. In this connection it has been observed that the wear imposed on print hammer mechanisms increases greatly at the higher printing speeds. Thus, wear which may be insignificant at printing speeds of up to 300 lines per minute may become a significant factor affecting the practical longevity of the print hammer mechanism when printing at 600 lines per minute is consistently required. Experience has shown that a significant amount of wear may occur on both the hammer spring and the primary pole piece tip at high printing speeds due to the frequent impacting of the primary pole piece tip by the free end of the hammer spring each time the hammer spring rebounds from printing a dot into the retract position. In time a crater-shaped recess begins to wear in the free end of the hammer spring, which recess can become as deep as 1.7 mils in places. Eventually, the hammer spring may actually fracture or may become difficult to pull into the retract position after each impact printing due to the increased air gap between the pole piece tip and the hammer spring defined by the crater. The pole piece tip is itself subject to wear. Attempts to minimize the wear by covering the pole piece tip with plastic materials and other elastomers have met with very limited success, apparently due to the rather substantial impact which occurs and the resulting tendency for such materials to wear out rapidly and require frequent replacement, thereby adding to the problem.
In addition to the wear problems which are aggravated by higher printing speeds in the print hammer mechanisms of the type shown in the Barrus et al patents, there are other operating characteristics of such mechanisms which are always subject to improvement. For example, the current required to energize the coil mounted on the primary pole piece so as to release the hammer spring is a significant factor in the overall current requirements of the printer. Any reduction in the current required to effect release of the hammer springs while at the same time achieving satisfactory operating characteristics of the print hammer mechanisms is a welcome improvement. Such a reduction in the current requirement is often accompanied by an improvement in the actual release characteristics of the hammer spring. It has also been observed in instances of high speed printing that portions of the hammer spring other than that which impacts the pole piece can undergo a type of metal corrosion due to abrasion and overall vibration of the mechanism. This has been observed to occur, for example, at the interfaces between the hammer spring and adjoining materials where the spring is mounted at its fixed end. Such metal corrosion also attributes to reduction in spring life.
Accordingly, it would be desirable to provide an improved print hammer mechanism.
It would furthermore be desirable to provide a print hammer mechanism which acts to reduce or minimize wear due to constant impacting of the hammer springs with the pole pieces of the magnetic structure.
It would furthermore be desirable to provide a print hammer mechanism having further potential or advantages in terms of other types of wear and in the actual operating characteristics of the mechanism.